I've had an '01 GMC with the 6.0L since it was new. It always sounded like this. You'd swear it was going to come apart with all of that tackety-tack-tacktac-a-tackety-tacka coming apart kind of noise. I now have 106k miles on it after pulling heavy campers, snowmobile trailers, horse trailers and other max GVWR loads at sometimes 5k RPM for miles at a time at 8-10k ft above sea level and it still runs like a top. This is not something to worry about, just ride that thing like was stolen.